The present invention generally relates to the field of chemical mechanical polishing (CMP). In particular, the present invention is directed to a CMP pad having an overlapping stepped groove arrangement.
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited onto and removed from a surface of a semiconductor wafer. Thin layers of conducting, semiconducting and dielectric materials may be deposited using a number of deposition techniques. Common deposition techniques in modern wafer processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) and electrochemical plating, among others. Common removal techniques include wet and dry isotropic and anisotropic etching, among others.
As layers of materials are sequentially deposited and removed, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., metallization) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful for removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches and contaminated layers or materials.
Chemical mechanical planarization, or chemical mechanical polishing (CMP), is a common technique used to planarize workpieces such as semiconductor wafers. In conventional CMP, a wafer carrier, or polishing head, is mounted on a carrier assembly. The polishing head holds the wafer and positions the wafer in contact with a polishing layer of a polishing pad within a CMP apparatus. The carrier assembly provides a controllable pressure between the wafer and polishing pad. Simultaneously therewith, a slurry, or other polishing medium, is flowed onto the polishing pad and into the gap between the wafer and polishing layer. To effect polishing, the polishing pad and wafer are moved, typically rotated, relative to one another. The wafer surface is polished and made planar by chemical and mechanical action of the polishing layer and polishing medium on the surface. As the polishing pad rotates beneath the wafer, the wafer sweeps out a typically annular polishing track, or polishing region, wherein the wafer surface directly confronts the polishing layer.
Important considerations in designing a polishing layer include the distribution of polishing medium across the face of the polishing layer, the flow of fresh polishing medium into the polishing track, the flow of used polishing medium from the polishing track and the amount of polishing medium that flows through the polishing zone essentially unutilized, among others. One way to address these considerations is to provide the polishing layer with grooves. Over the years, quite a few different groove patterns and configurations have been implemented. Conventional groove patterns include radial, concentric-circular, Cartesian-grid and spiral, among others. Conventional groove configurations include configurations wherein the depth of all the grooves are uniform among all grooves and configurations wherein the depth of the grooves varies from one groove to another.
It is generally acknowledged among CMP practitioners that certain groove patterns result in higher slurry consumption than others to achieve comparable material removal rates. Circular grooves, which do not connect to the peripheral edge of the polishing layer, tend to consume less slurry than radial grooves, which provide the shortest possible path for slurry to reach the pad perimeter under the forces resulting from the rotation of the pad. Cartesian grids of grooves, which provide paths of various lengths to the peripheral edge of the polishing layer, hold an intermediate position.
Various groove patterns have been disclosed in the prior art that attempt to reduce slurry consumption and maximize slurry retention time on the polishing layer. For example, U.S. Pat. No. 6,241,596 to Osterheld et al. discloses a rotational-type polishing pad having grooves defining zigzag channels that generally radiate outward from the center of the pad. In one embodiment, the Osterheld et al. pad includes a rectangular “x-y” grid of grooves. The zigzag channels are defined by blocking selected ones of the intersections between the x- and y-direction grooves, while leaving other intersections unblocked. In another embodiment, the Osterheld et al. pad includes a plurality of discrete, generally radial zigzag grooves. Generally, the zigzag channels defined within the x-y grid of grooves or by the discrete zigzag grooves inhibit the flow of slurry through the corresponding grooves, at least relative to an unobstructed rectangular x-y grid of grooves and straight radial grooves. Another prior art groove pattern that has been described as providing increased slurry retention time is a spiral groove pattern that is assumed to push slurry toward the center of the polishing layer under the force of pad rotation.
Research and modeling of CMP to date, including state-of-the-art computational fluid dynamics simulations, have revealed that in networks of grooves having fixed or gradually changing depth, a significant amount of polishing slurry may not contact the wafer because the slurry in the deepest portion of each groove flows under the wafer without contact. While grooves must be provided with a minimum depth to reliably convey slurry as the surface of the polishing layer wears down, any excess depth will result in some of the slurry provided to polishing layer not being utilized, since in conventional polishing layers an unbroken flow path exists beneath the workpiece wherein the slurry flows without participating in polishing. Accordingly, there is a need for a polishing layer having grooves arranged in a manner that reduces the amount of underutilization of slurry provided to the polishing layer and, consequently, reduces the waste of slurry.